It is well known in the art to use alkali metal and alkaline earth metal peroxide and superoxide chemicals as oxygen sources. Methods for releasing the oxygen have included introducing the chemical into a water containing reactor. For example, potassium superoxide is well known to react vigorously and almost instantly with water to evolve heat and oxygen with potassium hydroxide as the by-product.
Chemical source respirators and rebreathers use two chemicals almost exclusively; namely potassium superoxide (KO.sub.2) and anhydrous lithium hydroxide. In these applications, moisture in the breath is used as the activator. Both of these chemicals, which are in solid form, react with water exothermically to produce a hydrated metal hydroxide. LH does not liberate O.sub.2, but effectively absorbs CO.sub.2 in its hydrated form. It is limited to use in emergency rebreathers of short service life since no additional oxygen is provided. Potassium superoxide reacts with water to produce oxygen and hydrated potassium hydroxide. In a second reaction, the hydrate absorbs respired CO.sub.2 to form a carbonate/bicarbonate mixture. Surface crusts of carbonate sludge are formed with both of these chemicals due to agglomeration which interferes with continued water reaction and gas absorption.
Due to the formation of the carbonate sludge, the practical use of these chemical sources of oxygen has been shown to be inefficient. For instance, at high breathing rates (where moisture and CO.sub.2 levels are very high) there can be a 30 to 40% efficiency loss due to crusting of carbonates sludge. In practice, the efficiency loss is compensated for by using large excesses of these chemicals which is wasteful. In the case of potassium superoxide enough exotherm heat is created to require external heat exchangers on the potassium superoxide cannisters. However, this requires bulky oversized equipment. In addition, potassium superoxide is especially difficult to handle because its reaction with water is extremely fast and uncontrollable.
Because of the extremely rapid reaction of potassium superoxide and the like with water, many uses require that it be made into compressed briquette or blocks in order to control the rate of reaction. The increased density of the potassium superoxide in briquette form slows the diffusion of moisture into the chemical and extends the oxygen delivery time.
For instance, chemical oxygen masks based on potassium superoxide and the like use external cannisters with heat exchangers to hold the chemical, which is usually in the form of compacted blocks and arranged in a fashion that allows for air circulation. The cannisters have to be remote from the breathing mask because of their size, weight, and heat output. The inherent disadvantages of this type of remote unit are obvious. These units are cumbersome and wasteful of the potassium superoxide, which is expensive. Powdered potassium superoxide reacts instantly and efficiently with water and does not crust, but its reaction rate cannot be controlled.
Attempts to solve the above-mentioned problems which are inherent in these oxygen generating systems have been attempted in the prior art, but for the most part these systems have been cumbersome and only partially effective. For instance, U.S. Pat. No. 3,574,561 (Nickerson et al.) discloses an improved technique for controllably releasing oxygen from alkali metal peroxide or superoxide by spraying water upward onto the chemical contained in a downwardly directed elongated cartridge, and providing for gravity escape of the sprayed water plus hydroxide product This apparatus allows the solid state chemical to be progressively dissolved, and avoids the cleaning problems caused by the formation of carbonate sludge. The released oxygen is funneled into a delivery tube.
It has now been surprisingly found that microencapsulation of alkali superoxides, etc., in a water swellable capsule material provides particles with controllable reactivity which do not form carbonate crusts. In addition, these microcapsules exhibit less exothermic heat release when reacted with water, thus greatly reducing the need for separate heat exchangers. The microcapsule technique also can be used advantageously with anhydrous lithium hydroxide to inhibit carbonate crust formation during CO.sub.2 absorption by the hydrate.